It is generally known that automatic transmissions mounted on vehicles may include a hydraulic power transmission device such as a torque converter, coupled to a driving source such as an engine, and a transmission mechanism coupled to the hydraulic power transmission device and provided with a plurality of planetary gear sets (planetary gear mechanisms) and a plurality of friction engaging elements, such as clutches and brakes. The plurality of friction engaging elements are selectively engaged by a hydraulic control to achieve a plurality of gear positions with different gear ratios.
In recent years, there is a tendency to eliminate the hydraulic power transmission device because of the desire for an increasing number of gear positions, and a reduction of the weight, etc., of the automatic transmission. In this case, it is possible to realize a smooth start of traveling by carrying out a slip control of at least one of the friction engaging elements which is engaged at a first gear when the vehicle starts traveling, while avoiding an engine stall.
When carrying out the slip control of the friction engaging element engaged at the first gear when the vehicle starts traveling, since the brake that causes the hydraulic chamber not to rotate is better in controllability at the time of the engagement than the clutch which causes the hydraulic chamber to rotate, it is possible to carry out slip control of the brake engaged at the first gear when the vehicle starts traveling.
In an automatic transmission structured in this way, since the frequency of executing the slip control increases for the brake engaged at the first gear when the vehicle starts traveling, it is necessary to effectively reduce a generation of heat from a friction plate due to the slip control in order to secure a necessary durability.
In order to reduce the generation of heat from the friction plate, it is possible to increase the quantity of the hydraulic fluid for lubrication supplied to the friction plate to improve the cooling capability. However, in the brake in which a plurality of friction plates are disposed between an inner circumferential surface of the transmission case and an outer circumferential surface of a given rotary member accommodated in the transmission case, there is a possibility that the hydraulic fluid for lubrication stagnates near the inner circumferential surface of the transmission case to cause a drag between the friction plates to increases the rotational resistance.
On the other hand, for example, JP2016-090048A discloses a brake which is engaged at the first gear and is slip-controlled when the vehicle starts traveling. In this brake, a plurality of friction plates are disposed between an outer circumferential surface of a hub member coupled to a transmission case and an inner circumferential surface of a drum member coupled to a given rotary member.
FIG. 21 is a cross-sectional view illustrating a brake of a conventional automatic transmission. As illustrated in FIG. 21, a brake 200 includes a hub member 202 coupled to a transmission case 201, a drum member 204 coupled to a given rotary member 203, a plurality of friction plates 205 disposed between the hub member 202 and the drum member 204, and a piston 206 which engages the plurality of friction plates 205.
In the brake 200, a supply oil channel 207 for lubrication extends, as indicated by an arrow 208, radially inward of the transmission case 201 through the inside of a coupling member 209 coupled to the transmission case 201 and extends from one side in the axial directions to the other side through a spline part 210 formed in an outer circumferential surface of the hub member 202, to supply hydraulic fluid for lubrication to the plurality of friction plates 205.
The hydraulic fluid for lubrication receives a centrifugal force of the friction plates 205 which are spline-engaged with the drum member 204, and then, as indicated by an arrow 211, moves radially outwardly, and is supplied between the friction plates 205, to cool the friction plates 205 heated by the slip control. Then, the hydraulic fluid for lubrication which moved to an inner circumferential surface of the drum member 204 is prevented from, as indicated by an arrow 212, moving outward in the axial directions and stagnating by the rotation of the drum member 204.
In the brake disclosed in JP2016-090048A, as illustrated in FIG. 21, since the hydraulic fluid for lubrication is moved from one side in the axial directions to the other side through the spline part 210 formed in the outer circumferential surface of the hub member 202, the hydraulic fluid for lubrication may outflow radially outwardly as indicated by an arrow 213 before reaching the friction plates 205 to cause a reduction in the cooling efficiency of the friction plates 205.
On the other hand, inside the hub member, by forming the supply oil channel for lubrication so as to extend axially, and by providing a supply port and an introduction port in a part where the friction plates are disposed and a part where the coupling member is disposed, respectively, the hydraulic fluid for lubrication can efficiently be supplied to the friction plates to improve the cooling efficiency of the friction plates.
Therefore, in the automatic transmission provided with the brake where the friction plates are disposed between the hub member coupled to the transmission case and the drum member coupled to the given rotary member, when the supply oil channel for lubrication is formed in the hub member to supply the hydraulic fluid for lubrication from the hub member side, how to spline-engage the friction plates and how to couple the hub member having the supply oil channel for lubrication to the transmission case may be problems.
Since it is necessary to couple the hub member having the spline part with which the friction plates are spline-engaged to the transmission case so that the hub member receives a force inputted from the friction plates when engaging the brake to stop rotation of the friction plates, the hub member may be spline-engaged with the transmission case.
However, if the hub member is spline-engaged with the transmission case, a backlash may be produced in the circumferential directions of the transmission case at the coupling part of the hub member to the transmission case, which also induces backlash in the circumferential directions of the transmission case at a connection of the hub member and a valve body disposed in a lower part of the transmission case in the circumferential directions of the transmission case, and thus, it may become difficult to supply hydraulic fluid to the supply oil channel for lubrication of the hub member from the valve body.
On the other hand, although it is possible to press fit the hub member with which the friction plates are spline-engaged, into the transmission case so that the hydraulic fluid can easily be supplied to the supply oil channel for lubrication of the hub member from the valve body, there is a possibility in this case that the force inputted from the friction plates when engaging the brake cannot be received, and thereby a rotation stop cannot be achieved.